in Vol. 13 - May Issue - Year 2012
Higher Fatigue Resistance
Fatigue failures, also called fatigue fractures, mostly start as a small crack initiating structures and defects. Cracks grow mostly undetected until they finally cause a sudden forced rupture of the reduced remaining cross section.
The fatigue resistance i.e. the resistance to fatigue cracking of dynamically loaded components is largely determined by shape and structure of near surface layers of material.
More than 80% of fatigue cracks start at local surface-related structures. In many cases, the application of Poligrat-Processes can significantly reduce the risk of fatigue cracking and correspondingly increase lifetime several times.
Poligrat-Processes eliminate or reduce most of the near surface crack-initiating structures, which can be divided into two groups:
• Cracks initiated by micro notches. Essentially these are cracks at burrs and edges and sharp structures of the profile or crevices at grain boundaries. Stress concentration at the base of notches cause local concentration of tensile stresses with the risk that there will start a fatigue crack with the consequence of later rupture.
• Crack-initiating structures within a surface-related material layer with a thickness of several micro meters, which are caused by mechanical, thermal or chemical influence from fabrication. These layers, also called Beilby-layers, significantly differ from the original base material with respect to composition, structure and stresses. They often contain local tensile stresses, embedded foreign matter, coarse oxide grains and precipitations at grain boundaries, which are capable of initiating cracks.
Poligrat-Processes control and stress-free remove all damaged material layers from the surface with all the inherent stresses, foreign matter and other defects. They bring the undisturbed base metal with its best properties to the surface. At the same time, edges and surfaces are deburred and the microstructures smoothened and rounded. Existing micro cracks and sharp structures are removed or if they are larger in size, opened and rounded to a degree where there are no more notch factors. Revealing of cracks also provides a reliable quality control.
Poligrat-Processes are applicable widely independent of shape and size of parts and hardness of material. For most of the commonly used metals and alloys, industrially proven processes are available.
Long time experience shows that, on dynamically heavily loaded components, an increase of fatigue resistance by the factor 4 to 10 or more will result from the application of Poligrat-Processes. Typical applications are springs, turbine blades, hydraulic parts and nozzles.
Poligrat provides comprehensive advice for all applications and verification of achievable results by processing samples of original parts in their own laboratory.